1. Field of the Invention
This invention relates to PWM (pulse width modulated) switched mode power supplies and voltage converters with protection against overload currents.
2. Description of Related Art
Some voltage converters use pulse width modulation and a transformer to convert one DC voltage to another DC voltage. FIG. 1 shows a block diagram of a voltage converter 100 that converts a DC input voltage Vin at voltage level V1 to a DC output voltage Vout at voltage level V2. A control circuit 110 in voltage converter 100 connects and disconnects input voltage Vin to a primary coil 120 to generate a pulse width modulated (PWM) voltage across primary coil 120. (Voltage 210 in FIG. 2 shows a typical PWM voltage applied to primary coil 120 during normal operation.) The PWM voltage causes primary coil 120 to generate a time dependent magnetic flux which induces an AC voltage in a secondary coil 130. A voltage rectifier and filter 140 rectifies the AC voltage in secondary coil 130 and provides DC output voltage Vout to a load 150.
If load 150 develops a short or otherwise draws too much current, an overload occurs which could damage load 150 and voltage converter 100. Control circuit 110 typically contains an overload sensor and a duty cycle control circuit. The overload sensor senses peak current on an input side of voltage converter 100. The a duty cycle control circuit reduces the duty cycle of the PWM voltage when an overload current is sensed. Voltage 220 in FIG. 2 is an example of a PWM voltage with a reduced duty cycle.
FIG. 3 shows plots of output voltage Vout verses output current for a voltage converter such as voltage converter 100 in FIG. 1. Voltage plot 310 illustrates the desired behavior of voltage converter 100. In a range of currents from 0 to 100% of a rated current capacity of voltage converter 100, rectifier and filter 140 supplies output voltage Vout at level V2. When the output current reaches an overload level (in FIG. 3, overload current is between 105% and 115% of the rated current), control circuit 110 reduces the duty cycle of the PWM voltage driving primary coil 120. This reduces the power flow through voltage converter 100, and output voltage Vout drops. Ideally, output voltage Vout falls to zero which cuts off the current in load 150 and prevents circuit damage. In actual voltage converters, a varying PWM voltage applied to primary coil 120, even one with a small duty cycle, causes a non-zero output voltage Vout from rectifier and filter 140 and a non-zero current tail such as current tails 320, 321, or 322 which extend to high currents. In some applications, the current tail can contain sufficient power to damage load 150 and voltage converter 100.
In voltage converter 100, higher frequency PWM voltages permit the use of smaller components such as smaller primary and secondary coils 120 and 130 and smaller capacitors in voltage rectifier and filter 140, thus using higher frequency PWM voltages reduces the size and cost of voltage converter 100. Unfortunately, higher frequency PWM voltages also make reduction of duty cycle more difficult, and if the frequency is too high, the duty cycle cannot be reduced enough to prevent an overload current tail from damaging load 150 or voltage converter 100. Accordingly, a voltage converter is needed which operates at high PWM frequency and which provides a better voltage cutoff to prevent damage caused by overload currents.